How to trigger macro when combobox change

Hello,Can anyone help me in this issue, i have the below function to run whenever we open report:
function __open(){
$start = new DateTime(activeworkbook()->sheets('ChooseStep')->Range('J10')->value);
$end = new DateTime(activeworkbook()->sheets('ChooseStep')->Range('J12')->value);
// otherwise the end date is excluded
$end->modify('+0 day');
$interval = $end->diff($start);
// total days
$days = $interval->days;
// create an iterateable period of date (P1D equates to 1 day)
$period = new DatePeriod($start, new DateInterval('P1D'), $end);
// best stored as array, so you can add more than one $holidays = array('2012-09-17');